Yes, you can have multiple ID selectors to target an element in Javascript. You can use the querySelectorAll() method to select multiple ids like this:

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const elements = document.querySelectorAll('#id1, #id2, #id3');

In CSS, you can add a class to an element using the class attribute. The class you wish to assign should be added to the class attribute, with multiple classes separated by spaces. See the example below:

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<div id="example" class="style1 style2">
  Some content
</div>

You can also assign a class to an element using the class selector in your CSS file. The corresponding class name should be added after the selector, preceded by a ., like the example below:

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.style1 {
  font-weight: bold;
  color: blue;
}

You can use the CSS class selector to style an element with multiple classes. The class selector is denoted by a period . followed by the class name.

For example, given the following HTML:

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<div class="block red color-1">
</div>

You can use CSS to style the element like this:

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.block {
  background-color: yellow;
}
.red {
  color: red;
}
.color-1 {
  color: blue;
}

This would render the HTML element with a yellow background, red text and blue text (overriding the style set by the red class).

In CSS, you can make an element on your page expand when you hover over it. To do this, you can use the :hover selector.

Here's an example of how you could use :hover to expand a button element:

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button {
  width: 70px;
  height: 30px;
}

button:hover {
  transform: scale(1.2);
}

In this example, the transform: scale(1.2); line tells the browser to scale the element up to 120% of its original size when the mouse hovers over it.

To add a style to a <li> element without interfering with the styling of an <h1> element, you can use the div selector. With the div selector, you can specify what styling to apply to the <li> elements without conflicting with the <h1>.

For example in CSS, you can use the following code:

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div .list-item {
  color: #ff0000; 
}

This will add styling to any <li> elements that are a child of a <div>. Here, color: #ff0000; makes all the text in the <li> element red.

More information about the div selector can be found in this guide.

document.querySelector() is a method in JavaScript that is used to query a document for an element and return the first element within the document that matches the specified selector or group of selectors. It returns the element object if there is a match, or null if there is no match. Generally, it is used to select a single element from a document, similar to how getElementById() works but more powerful. For example, the following code selects the first <p> element in the document:

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let firstParagraph = document.querySelector('p');

Reference: MDN